Over the past few years, wide use has been made of ionomer resins in golf ball cover materials, also referred to hereinafter as “cover stock”. Ionomer resins are ionic copolymers composed of an olefin such as ethylene in combination with an unsaturated carboxylic acid such as acrylic acid, methacrylic acid or maleic acid, wherein the acidic groups are partially neutralized with metal ions such as sodium, lithium, zinc or magnesium ions. They have excellent characteristics such as durability, resilience and scuff resistance.
Ionomer resins account for most of the cover stock resin in current use. A variety of improvements have been made thereon because golfers are always on the lookout for golf balls having a high rebound and excellent flight characteristics.
Related improvements taught by the prior art (see U.S. Pat. Nos. 5,312,857, 5,306,760, and International Application WO 98/46671) include cover stocks in which a large amount of metallic soap is added to the ionomer resin to improve the cost and rebound characteristics of the ionomer cover stock.
These cover stocks, however, fall far short of practical levels. During injection molding, the metallic soap in the cover stock decomposes and vaporizes to generate a large amount of fatty acid gas, often causing molding defects. In addition, gas constituents settle on the surface of the molded part to greatly lower the paintability thereof. Moreover, although such cover stock in which a large amount of metallic soap has been added to the ionomer resin does exhibit a rebound which is about the same as or better than that of metallic soap-free ionomer cover stock having the same degree of hardness, the improvement in rebound is not all that large. Indeed, depending on the type of metallic soap used, the moldability and rebound of cover stock may in fact be severely compromised.